W3Schools
http://www.w3schools.com

WebReference
http://www.webreference.com

WebMonkey
http://www.webmonkey.com

HTML Goodies
http://www.htmlgoodies.com

HTML Cheat Sheet:
http://hotwired.lycos.com/webmonkey/reference/html_cheatsheet/

HTML Color Codes:
http://hotwired.lycos.com/webmonkey/reference/color_codes/

HTML Special Characters Reference:
http://hotwired.lycos.com/webmonkey/reference/special_characters/

Creating HTML Frames:
http://hotwired.lycos.com/webmonkey/authoring/frames/index.html

Creating HTML Tables:
http://hotwired.lycos.com/webmonkey/authoring/tables/index.html

Cascading Stylesheets (CSS):
http://hotwired.lycos.com/webmonkey/authoring/stylesheets/index.html

DHTML Reference:
http://msdn.microsoft.com/library/default.asp?url=/workshop/author/dhtml/reference/dhtmlrefs.asp

XML:
http://hotwired.lycos.com/webmonkey/authoring/xml/index.html

The WWW Consortium (straight from the horse's mouth):
http://www.w3.org/